The present invention relates to a space diversity system for the transmission of digital microwaves.
In space diversity systems heretofore proposed, composite outputs are provided either by a method relying on inphase combination or a method relying on switching. Of the two known methods, the former is predominant today because the latter is unsuitable for digital transmission systems due to a phase difference between two antenna outputs which would cause switching to bring discontinuity into phases.
Recently, high capacity digital transmission with high stability as typified by 16 QAM (Quadrature Amplitude Modulation) has been put to practical use. In parallel with this trend, the role initially assigned to the space diversity system for insuring receive signal levels is being replaced by a new role for removing interference waves, which would creat distortions in direct waves. The interference cancellation type space diversity system implements the demand for the new role. A radio receiving apparatus using such a system is disclosed in U.S. Pat. No. 4,326,294 issued Apr. 20, 1982. A drawback inherent in this prior art apparatus is that, where two antenna inputs are quite similar to each other, the combined signal level becomes far lower than the noise level tending to bring about an error in identification.